This invention relates to liquid ring compressors, also known as liquid ring vacuum pumps.
Known liquid ring compressors consist of a substantially cylindrical casing within which an impeller provided with a plurality of blades can rotate. The axis of rotation of the impeller is parallel to the axis of the cylindrical casing but offset from it.
The cylindrical casing is closed at its two ends, at least one being closed by a removable distribution plate containing suction and discharge apertures for the intake gas, and the feed aperture for the operating liquid (usually water).
The distribution plate is maintained in position by a relative cover fixable to the cylindrical casing of the compressor to also form the connection between said suction and discharge apertures and the gas intake and discharge duct respectively.
Between that front surface of the impeller which faces the distribution plate and the corresponding surface of this latter there is a very small gap to provide a seal for the conveyed gas.
During the operation of such a compressor, the motor-driven impeller rotates the operating liquid which partly fills the cylindrical casing. Under the action of centrifugal force the operating liquid assumes the form of a ring (from which the liquid ring compressor derives its name) which is concentric with the cylindrical casing of the compressor.
During one complete revolution of the impeller shaft (one cycle) each pair of consecutive impeller blades cooperates with the surface of the formed liquid ring to define a chamber, known as the transport chamber, which varies in volume. During the initial 180 degree stage of the cycle, the blade immersion into the liquid ring gradually decreases, so that the volume of the transport chamber increases from zero to a maximum. During the final 180 degree stage of the cycle, the blade immersion into the liquid ring gradually increases, so that the volume of the transport chamber decreases from the maximum to zero. Consequently during the initial stage of the cycle, by connecting an individual transport-chamber for at least a part of said initial stage to the suction aperture provided in said distribution plate, gas is drawn in through the suction aperture, this having an appropriate shape. During the final stage of the cycle, by connecting an individual transport chamber to the discharge aperture, also provided in the distribution plate, the gas discharges through the discharge aperture, this also having an appropriate shape.
From the time of invention of liquid ring compressors (several decades ago) to the present-day, the relative distribution plates have been constructed of cast iron or stainless steel by casting followed by machining on a machine tool by chipping (turning).
Distribution plates constructed in this manner cannot however reliably and reppeatably achieve the geometrical tolerance required to ensure correct operation (high efficiency) of a compressor of this type. It has therefore been necessary in practice to use compromise design solutions which penalize the compressor performance, in order to achieve acceptable production costs.
Examining in greater detail the intrinsic limits of distribution plate construction by casting, it can be stated that:
a) the actual characteristics of the casting process and of the equipment used in it limit the designer in his choice of the most suitable shape and dimensions for the suction and discharge apertures;
b) even though attempting to reduce to a minimum the thickness ofxe2x80x94distribution plates obtainable by casting, the thickness is still such as to require the formation of rounded edges or chutes for the suction and discharge apertures in order to achieve an acceptable outflow coefficient, this complicating the casting process and the equipment used in it;
c) because of the nature of the casting process itself, it is impossible to ensure that the suction and discharge apertures will have the correct position or shape.
Summarizing, besides not being able to guarantee that the distribution plate obtained has the correct geometry, the casting process is costly especially for stainless steel casting, and the subsequent machining of the distribution plate by machine tools is complicated and costly. In particular it is difficult to maintain within the required tolerance the planarity of that distribution plate surface facing the impeller, with the result that generally the gap between the impeller and distribution plate is greater than the ideal, with consequent penalization of the compressor performance, particularly during low pressure operation.
One attempt to overcome these drawbacks was to construct the distribution plates of ceramic. Although this solution gives good results in terms of aperture geometrical tolerance and plate planarity, it, is very costly. Moreover ceramic plates are delicate because of fragility, being subject to breakage both during compressor assembly and during compressor, operation, particularly if large temperature differences are present.
The object of the invention is to overcome the aforementioned drawbacks of known liquid ring compressors.
This object is attained by the liquid ring compressor of the invention, characterized in that the distribution plate (in the case of compressors with a single distribution plate) or the two distribution plates (in the case of compressors with two distribution plates) are plates of small thickness (compared with known plates) constructed from sheet steel.
As the plates are thin, the suction and discharge apertures can have sharp edges. These apertures can be formed by punching or by plasma or laser cutting.
The fact of using a thin steel sheet for constructing the distribution plates and of forming-the suction or discharge apertures by plasma or laser cutting or by punching ensures that the tolerances both of the planarity of that distribution plate surface facing the impeller and of the position and shape of the suction and discharge apertures fall within limits which do not appreciably penalize compressor performance, all at a lower cost than that of known liquid ring compressors. It should also be noted that by using the aforesaid cutting methods, apertures with very small radii of curvature can be formed in the distribution plates, so giving the designer total freedom in choosing the most suitable shapes for the suction and discharge apertures, and ensuring repeatability of the shape chosen for these apertures.
Moreover the use of plates of very thin sheet steel (up to a few millimeters) means that the suction and discharge apertures are of a thin wall type so that they do not require their edges to be of complex geometry as in the case of known cast plates, in which because of the significant plate thickness it becomes necessary to provide lead in roundings or chutes to ensure a good outflow coefficient for the apertures.
Planarity and stability of that plate surface facing the impeller is ensured by the intrinsic constancy of the thickness of the steel sheet from which the plate is formed, and by the planarity of the surface of the relative compressor front cover with which the distribution plate comes into contact. To obtain good distribution plate stability during compressor operation (so preventing any deformation perpendicular to the plate), the relative cover is adequately ribbed to adequately support the plate.
Conveniently the ratio of useful outer diameter of the distribution plate to its thickness is greater than 50.